The present invention relates generally to materials suitable for use in electronic devices. More specifically, the invention relates to materials and films comprising same having an improved elastic modulus and a low dielectric constant expressed through a higher normalized wall elastic modulus and to methods for making same.
There is a continuing desire in the microelectronics industry to increase the circuit density in multilevel integrated circuit devices such as memory and logic chips in order to improve the operating speed and reduce power consumption. In order to continue to reduce the size of devices on integrated circuits, it has become necessary to use insulators having a low dielectric constant to reduce the resistance-capacitance (“RC”) time delay of the interconnect metallization and to prevent capacitive crosstalk between the different levels of metallization. Such low dielectric materials are desirable for premetal dielectric layers and interlevel dielectric layers.
Typical dielectric materials for devices with 180 nm line width are materials with a dielectric constant between about 3.8 and 4.2. As the line width decreases, the dielectric constant should also be decreased. For example, devices with 130 nm line width require materials with a dielectric constant between about 2.5 and 3.0. Extremely low dielectric constant (“ELK”) materials generally have a dielectric constant between about 2.0 and 2.5. Devices with 90 nm line width require materials with dielectric constants less than 2.4. According to the 2001 International Technology roadmap for Semiconductors (ITRS) interconnect roadmap, the projected dielectric constant requirements for interlevel metal insulators will be less than 2.1 for the 65 nm node, less than 1.9 for the 45 nm node, less than 1.7 for the 32 nm node, and less than 1.6 for the 22 nm node.
The dielectric constant (κ) of a material generally cannot be reduced without a subsequent reduction in the mechanical properties, i.e., modulus, hardness, toughness, of the material. Mechanical strength is needed for subsequent processing steps such as etching, CMP (“Chemical Mechanical Planarization”), and depositing additional layers such as diffusion barriers for copper, copper metal (“Cu”), and cap layers on the product. In some of these processes, temperature cycling of multiple layers may induce stresses due to the thermal coefficient of expansion mismatch between the different materials thereby causing cracking or delamination. Surface planarity is also required and may be maintained through controlling processing parameters such as those during the film formation process and also through CMP. Mechanical integrity, or stiffness, compressive, and shear strengths, may be particularly important to survive CMP. It has been found that the ability to survive CMP may be correlated with the elastic, or Young's, modulus of the material, along with other factors including polishing parameters such as the down force and platen speed. See, for example, Wang et al., “Advanced processing: CMP of CU/low-κ and Cu/ultralow-κ layers”, Solid State Technol., September, 2001; Lin et al., “Low-k Dielectrics Characterization for Damascene Integration”, International Interconnect Technology Conference, Burlingame, Calif., June, 2001. These mechanical properties are also important in the packaging of the final product.
A number of processes have been used for preparing low dielectric constant films. Chemical vapor depostion (CVD) and spin-on dielectric (SOD) processes are typically used to prepare thin films of insulating layers. Other hybrid processes are also known such as CVD of liquid polymer precursors and transport polymerization CVD. A wide variety of low κ materials deposited by these techniques have been generally classified in categories such as purely inorganic materials, ceramic materials, silica-based materials, purely organic materials, or inorganic-organic hybrids. Likewise, a variety of processes have been used for curing these materials to decompose and/or remove volatile components and substantially crosslink the films such as heating, treating the materials with plasmas, electron beams, or UV radiation.
Since the dielectric constant of air is nominally 1.0, one approach to reducing the dielectric constant of a material may be to introduce porosity. Porosity has been introduced in low dielectric materials through a variety of different means. A dielectric film when made porous may exhibit lower dielectric constants compared to a dense film, however, the elastic modulus of the film generally decreases with increasing porosity. Consequently, it may be impractical to use these low dielectric compositions due to the trade-off in dielectric constant with elastic modulus. Furthermore, it is often difficult to identify improved low dielectric compositions due to this trade-off in dielectric constant with elastic modulus.
The relationship between dielectric constant and porosity within a material has been approximated in numerous ways, see, for example, Kingery et al., Introduction to Ceramics, John Wiley & Sons, Inc., 1970, pp. 947–948 (“Kingery”), due to the difficulties in measuring the actual dielectric constant of the material. The physical models described in Kingery consider the dielectric material as a being a two-phase mixture of ideal dielectrics. For the purposes of approximating porosity, air having a dielectric constant of 1.0 may be viewed as one of the components of the mixture. These mixtures can be viewed in several ways. One way is to view the mixture as layers of materials having layers parallel to the capacitor plates. Another way is to view the layers of material as normal to the capacitor plates. Yet another way is to apply the logarithmic mixture rule which gives values intermediate between the parallel and normal extremes. In a still further way, Maxwell has derived a relationship for a dispersion of spherical particles in a matrix, which approaches the logarithmic mixture rule when the dispersed phase has a dielectric constant much higher than the matrix material, but very close to the normal layered model when the dispersed phase has a dielectric constant much lower than the matrix material.
Like the relationship with dielectric constant, the relationship between elastic modulus and porosity for a material comprising multiple phases has also been approximated in numerous ways, see, for example, Kingery at pp. 773–777. In two-phase systems, the overall modulus for a material that is a mixture falls between the elastic moduli of the low and high modulus components. Kingery describes a variety of different models, such as Voigt, Reuss, and Hashin and Shtrikman, which attempt to define the upper and lower bounds of moduli for a mixture. The extreme case in adding a low modulus material as a second phase to produce a mixture with pore spaces that have zero bulk modulus. In this case, MacKenzie has derived an expression to represent the change in elasticity for closed pores in a continuous matrix up to porosities of about 50%.
The reference, Day, et al., “The Elastic Moduli of a Sheet Containing Circular Holes”, J. Mech. Phys. Solids, Vol. 40, No. 5, pp. 1031–51, 1992 (“Day”), describes a method using computer simulation techniques to obtain the elastic moduli of a matrix containing circular holes. Day applies the results of the simulation data for a regular array of circular holes in a triangular arrangement to an interpolation formula that incorporates the known results, i.e., the initial slope, the percolation concentration, and the critical exponent, as well as a free fitting parameter to obtain the relative Young's Modulus E/E0, where E is the Young's modulus of the overall material, and E0 is the Young's modulus of the matrix.
The reference Golden et al., “Designing Porous Low-κ Dielectrics”, Semiconductor International, May 2001 (“Golden”), describes applying the Bruggeman effective medium approximation model to predict the effect of porosity on dielectric constant. The Bruggeman model predicts that a host matrix with a lower κ value than SiO2 would need less porosity to achieve the lowest target κ value. For example, up to 50% porosity is needed to obtain κ=2.0 starting from a matrix material having a κ=4.2 (the value for dense silica) whereas only 22% porosity is needed in a κ=2.5 matrix material. Less porosity may be better for maintaining mechanical properties if the mechanical properties of the dense oxide and the κ=2.5 material are equivalent. Realistically, however, the elastic modulus for the κ=2.5 material is lower than the elastic modulus of the dense oxide. Although the modulus of the κ=2.5 material is not decreased in the same amount by the introduction of 22% porosity as is the modulus of the dense oxide by the introduction of 50% porosity, if the modulus of the κ=2.5 material is low to begin with, the overall modulus after the introduction of 22% porosity could be lower than the overall modulus of the dense oxide after the introduction of 50% porosity. Consequently, it is unclear which of the two materials at a κ=2.0 will have better mechanical properties. This is a critical failing in the prior art: the inability to understand and quantify the trade-off between κ and modulus to be able to identify a material with improved performance.
Some designers of low dielectric materials have attempted to correlate the relationship between dielectric constant, elastic modulus, and porosity in order to achieve a low dielectric material with good mechanical properties. The reference, Ramos et al., “Mechanical and Electrical Properties of NANOGLASS™ Nanoporous Silica as a Function of Porosity” (“Ramos”), found on the website www.honeywell.com, discloses that the modulus for the NANOGLASS™ materials varies proportional to (κ−1)x where x=2.5. The κ for these materials was adjusted by changing the amount of solvent. However, these results were empirically derived for a given set of materials and are not generally extendible to other classes of materials.
The reference, Bremmer, “A New Class of Insulating Materials: Emergence of ultralow-κ”, Solid State, Technology, September 2001 (“Bremmer”), describes two-component models to approximate properties of porous dielectric materials. The dielectric constant was predicted using a two-phase parallel capacitance model and the modulus of elasticity was approximated by a power function of film porosity. Bremmer provides plots of E vs. κ for 3 different porous materials and dense silica that typify the matrix materials. Bremmer did not provide the values for Ematrix, κmatrix, and m, the power coefficient to approximate E degradation. Further, Bremmer did not teach that one value, or figure of merit, could be used to characterize each E vs. κ curve.
Another consideration in the production of low dielectric materials and the resultant film is the level of metal impurities present in the material. In order for a low dielectric film to be suitable for Integrated Circuit (IC) fabrication, it is desirable that the film has a controlled level of impurities. In other words, the film should be deposited using ingredients that have minimal levels of nonvolatile impurities that may be harmful in silicon oxide-based insulator films in microelectronic devices. In the IC industry, it is well known that alkali metal ions such as sodium and potassium should be excluded from silicon dioxide films used as metal oxide semiconductor (“MOS”) transistor insulators and multilevel interconnection insulators. These positively charged ions might become mobile when exposed to electric fields and drift away from the positively biased film interface and toward the negatively biased film interface causing capacitance-voltage shifts.
Some commercially available chemical reagents used in the production of low dielectric films contain alkali metal impurities. These impurities may result from residual levels of catalyst used in the manufacture of the chemical precursor reagents. Ratios of 0.005–0.05:1 mol of NaOH, KOH, or NaOCH3 to alcohol are frequently used in the base-catalyzed ethoxylation of aliphatic alcohols, alkylphenols, and fatty acids. See, e.g., Lynn et al.,“Surfactants”, Kirk-Othmer Encyclopedia of Chemical Technology, John Wiley & Sons, Inc., (1997). For example, the use of 0.005 mol NaOH per mol of alcohol in the production of TRITON™ X-114, an alklyphenol ethoxylate with an average 7.5 moles of ethoxylate per mole of alcohol, may result in 214 ppm of sodium in the final product. Such levels of residual catalytic impurities are often of little consequence in typical applications of these chemicals because the surfactant is often used at such low levels that the catalytic impurities imparted by the surfactant become insignificant in the final formulation. A polymer such as polyethylene glycol (PEG) may be made using different catalyst systems depending on the desired molecular weight. For molecular weight below 20,000, base or the Na+ or K+ alkoxides of methanol or butanol are used as the catalyst. See, for instance, Glass, J. E. “Water-Soluble Polymers”, Kirk-Othmer Encyclopedia of Chemical Technology, John Wiley & Sons, Inc. (1988). Solvents, like surfactants, can also contain residual catalytic impurities. For instance, the formation of ethers, such as propylene glycol propyl ether (PGPE), through the reaction of propylene oxide with an alcohol, is often base-catalyzed when high selectivity to the primary alkyl ether over the secondary ether is desired which can result in residual impurities. See, for instance, Brown, et al., “Glycols: Ethylene Glycol and Propylene Glycol”, Kirk-Othmer Encyclopedia of Chemical Technology, 3rd ed., John Wiley & Sons, N.Y., (1980), Vol. 11, p 953. A further source of impurities may result from an inattention to detail, such as packaging or handling outside a clean room, because such stringent purity requirements are not needed for typical applications.
Alkali metal impurity specifications for chemical precursor solutions for integrated circuit applications typically set the allowable impurity levels to approximately 20 parts per billion maximum for each type of alkali metal and less than 50 ppb total. To meet these limits, the material supplier to the IC industry may purify the surfactants. The reference, EP 1,142,832, assigned to the assignee of the present application, discusses how the dielectric and mechanical properties of the resulting films may be adversely affected by the purification of surfactants used as porogens in the film-forming mixture. The reference, US 2002/0045693, discusses how the dielectric properties of the resulting films may be adversely affected by the purification of reagents even if surfactant is not present.
Yet another concern in the production of low dielectric films is the processing or cycle time. The cure or anneal step, in which the coated substrate is typically heated to decompose and/or remove volatile components and substantially cross-link the film, is a significant source of production bottlenecks. The majority of low and ultralow dielectric films currently made have a cure step which ranges from greater than 30 minutes to 2 hours. Consequently, reduction of the cure step time would reduce the overall process time and achieve higher manufacturing throughput.
Another concern is the overall thermal budget. Various components of IC devices such as Cu metal lines can only be subjected to processing temperatures for short time periods before their performance deteriorates due to undesirable diffusion processes. Most processes for preparing silica-based low κ films require curing steps at temperatures of 450° C. or higher and times of 30 minutes or longer. Significant advantages could result if the curing step could be carried out at significantly lower temperatures and or shorter times.
Accordingly, there is a need in the art to provide improved dielectric materials having low dielectric constant and sufficient mechanical strength. To achieve that end, there is also a need in the art to provide a means to correlate dielectric constant, porosity, and elastic modulus to identify and develop low dielectric materials. There is also a need in the art to provide dielectric materials and films that have relatively low metal content yet still maintain the beneficial properties, i.e., lower κ and higher modulus, that high levels of metals may impart. Further, there is a need in the art to provide processes for making low dielectric films at relatively low temperatures and relatively short cycle times.
All references cited herein are incorporated herein by reference in their entirety.